The resurgence of direct current
David Cross, HVDC initiative leader
Advances in power electronic converters makes high-voltage direct current feasible for long-distance transmission. The proponents of alternating and direct current power supply battled for supremacy in the US in the late 19th century. It was known as the War of the Currents.
The two main protagonists were Thomas Edison and Nikola Tesla. Alternating current (AC), which was backed by Tesla, emerged the winner. A key factor in why it won, and why all cities today are powered by AC-based infrastructure, is that transformers do not work well with direct current (DC).
But today DC is enjoying something of a resurgence, especially when it comes to the transmission of large amounts of power over long distances, and to incorporate renewable power into electricity grids.
Depending on how far the electricity travels between the point of generation and the customers, up to 15% of the power can be lost along the way. So, a transmission line to serve a city of 1M people only really provides enough power to serve 850,000. Today, thanks to high-voltage direct current (HVDC) technology, grid operators can efficiently transmit power underground, underwater and across countries from remote areas where renewables like wind, solar and hydro are usually generated to where it is needed.
Reduced power losses
Transmitting power over distance is essentially a balancing act. A high voltage is required to send power a long way, but AC transmission is inefficient over long distances via overhead line and cable. By contrast, power losses in DC transmission are about one tenth of AC.
However, DC transmission infrastructure is relatively expensive to build, not least because it requires large and complicated boxes of electronics at either end to convert it to or from AC and to step the voltage up and down. But DC transmission towers are smaller and carry fewer wires than AC equivalents, so are cheaper, require less right of way and have a smaller visual impact. Broadly speaking, DC is the better option to transmit more power over a long distance.
HVDC transmission has been around for almost 100 years, but its use until recently has been limited. Advances in the electronics that underpin the converters at either end are increasingly making HVDC a solution of choice.
Offshore windfarms and subsea cable are key drivers
The key innovation underpinning the resurgence of HVDC over the past 20 years is insulated gate bipolar transistors (IGBTs), which are being used for voltage source converters (VSC). Converting AC to DC and back again requires semiconductor switches, and the introduction of IGBT transistors has made converters more controllable.
The offshore windfarm market is driving the market for VSC technology. Germany has been a pioneer in this area – initially in a bid to reduce carbon emissions and more recently in response to the 2011 Fukushima nuclear disaster, which prompted the government to pledge to shut the country’s 17 nuclear reactors by the end of 2022.
An historical problem, particularly for offshore windfarms, had been the size of the valve halls needed at either end of subsea cables, which made the connection expensive. Swedish technology company ABB developed a system called HVDC Lite. It incorporates the latest VSC/IGBT switching equipment, providing faster and more efficient conversion with fewer losses. It is also a more compact HVDC convertor and can be installed more easily on an offshore platform. Other manufacturers are now producing similar systems.
For subsea cables, DC typically becomes financially viable over transmission lengths of more than 70km. For overhead power lines, the tipping point in favour of DC is around 150km.
Increasing market share
As the power electronic and DC cable technology develops, both VSC and line commutated converter (LCC) systems will find a growing market. HVDC is well suited to the rising trend for electricity to be traded across international borders, particularly in Europe. A 2GW HVDC link using the traditional thyristor type LCC system was installed between England and France in 1986, mainly to deliver French nuclear power. A 1GW project (known as Eleclink) to run a DC cable through the Channel Tunnel is currently under construction.
More long-distance interconnectors from Europe to the UK are under development and construction, including to Scandinavia to exploit surplus hydropower. A link to Iceland to tap into its geothermal energy resource is also on the cards. It’s more than 1600km, but advances in HVDC converter and cable technology make it technically viable to transmit energy. HVDC offers other advantages. It improves power system stability and can be used to create a hybrid approach also using AC.
Embedded HVDC offers opportunities to strengthen grids with reduced environmental impact compared with upgrading AC transmission lines. And asynchronous HVDC connections are often the only practical way to join systems operating at different frequencies or where it is not possible to make a direct link.